Freezing rain event 1415 December 2005 Central South Carolina

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Freezing rain event 14-15 December 2005Central
South Carolina
Tony Petrolito NWS Forecast Office Columbia SC
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Introduction

• Initially it appeared temperatures would be just
  warm enough for all rain. Cold dry high pressure
  to the north.
• A light freezing rain event developed across the
  piedmont and north midlands after midnight 15
  December 2005.
• Icing was confined to elevated surfaces (trees
  power lines). Soil temperatures well above
  freezing.

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Introduction-Cont.

• During the morning 14 December, WFO CAE
  forecasters determined (based on observational
  trends) that the numerical models and model
  output statistics (MOS) were incorrectly
  forecasting surface temperatures too warm and
  were also not accounting for dry air near the
  surface.
• WFO CAE forecasters decided to issue a winter
  weather advisory for freezing rain for the
  northern portion of the county warning forecast
  area.

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Synoptic overview
GFS 18-hr forecast 500mb geopotential heights
(m x 10), vorticity (s-1) and wind (kt) valid
1200 UTC 15 December 2005.
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NCEP/HPC observed mean sea level pressure (mb)
and frontal analysis valid 1200 UTC 15 December
2005. (weak CAD with surface temperatures near
0oC across north central S.C.)
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MSAS mean sea level pressure analysis (mb) and
METAR observations standard plot valid 1900 UTC
14 December 2005.

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GFS 00-hr forecast precip water and precip water
observed plot valid 1200 UTC 14 December 2005.
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NAM 00-hr forecast precip water and precip water
observed plot valid 1200 UTC 14 December 2005.
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Laps sounding at CAE valid 1800 UTC 14 December
2005. note deep cold and dry layer below 600 mb.
The sounding saturated above 600 mb suggesting
extensive mid to high level clouds limiting solar
insolation during the afternoon.
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ACARS sounding near CAE valid 1855 UTC 14
December 2005.
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Mesoscale overview
MSAS mean sea level pressure (mb), surface
temperature oF and surface wind (kt) valid 2100
UTC 14 December 2005.
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NAM 3-hr forecast mean sea level pressure (mb),
surface temperature oF and surface wind (kt)
valid 2100 UTC 14 December 2005.
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CAE GFS MOS GUIDANCE 1200 UTC 14 December 2005
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CAE NAM MOS GUIDANCE 1200 UTC 14 December 2005
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NAM 24-hr forecast 295K pressure (mb), 295K
pressure advection (um/s, and wind (kt) valid
1200 UTC 15 December 2005. WFO CAE QPF forecasts
were in the 0.25 to 0.50 inch range for the 12-hr
period ending at 1200 UTC.
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Fa1 if all the freezing takes place on elevated
surfaces Fa0.5 if freezing occurs on ground with
cold bare ground
icing (mm)0.05dTdP/Fa assuming adequate QPF
dTfreezing point depression (oC) dPdepth of
surface based sub-freezing layer(mb)
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Winter weather advisory (issued at 1535 LST 14
Dec.)

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Summary

• Light to occasionally moderate freezing rain
  event developed after midnight 15 December.
• With soil temperatures above 0oC, icing was
  confined to elevated surfaces. Local application
  based on CSTAR research suggested icing potential
  around 0.10 inches was possible. Spotter reports
  confirm icing accretion up to 0.10 inches on
  trees and power lines.
• WFO CAE forecasters demonstrated excellent
  situational awareness by recognizing that the
  model forecasts of surface temperatures and dew
  points were not panning out in correspondence
  with observations.
• It is important for forecasters to routinely
  compare model soundings with ACARS, Laps, and
  other observational data sets.